Gas pipe ignitors are used in industrial and utility scale boilers to bring the boiler temperature up before introducing the main fuel and also to light the main fuel once it is introduced. Additional uses include operation during periods of high demand to increase the heat rate of the boiler. One known configuration of the gas pipe ignitor uses a stabilized pilot flame to ignite and stabilize a larger, non-premixed diffusion primary flame at the flame end of the ignitor. Combustion air for the pilot flame is supplied through the ignitor, while combustion air for the primary flame is scavenged from the boiler environment. High capacity gas ignitors may conventionally use two separate fuel pipes for the delivery of gas. One pipe is used for the pilot gas and primary gas, while the other is used for boost gas. The pilot/primary gas pipe contains a number of small weep holes, positioned near a spark discharge for ignition. This pipe has an orifice mounted in the discharge end that is used to create the pressure differential necessary to force gas out of the weep holes while still allowing the primary gas jet to be discharged from the end. In cases where a greater firing rate is desired, the boost fuel pipe is activated. In that case, the boost fuel pipe discharges fuel at the same location as the end of the pilot/primary gas pipe. Both of the pipes are located inside of the air supply pipe which carries combustion air for the pilot flame. Additionally, a spark rod used for ignition and a separate flame detector rod are mounted inside of the air supply pipe.
Approximately 35% of the internal volume of the air supply pipe is occupied by the fuel pipes and these other fittings resulting in a high velocity turbulent air flow through the air supply pipe and significant drag losses owing to the high surface area of the internal pipes and fittings. Further, structures within the air supply pipe result in high frictional losses exacerbated by the high upstream air velocity.
The limit on the firing capacity of the ignitor depends on a number of key variables. The heat input from the bluff body stabilized pilot flame dictates the lift-off and blow-off characteristics of the main jets. The size of the pilot flame is dependent on how much combustion air can be supplied through the ignitor as well as on the size and geometry of the recirculation zone. Also, the outlet diameters of the main jets determine the exiting velocity of the gas for a given flow rate. With limitations on the air pressure available for the pilot combustion air, it becomes necessary to reduce the flow induced frictional losses caused by the presence and location of pipe and fittings as well as other combustion supporting structure in the air supply pipe.
U.S. Pat. No. 5,865,616 to George describes a premix gas burner having a main gas tube, a pilot tube, and an ignitor. This conventional burner is representative of the complexity and number of conduits for air and fuel supply that may be comprised in a burner.
An object of the present invention is to provide a gas pipe ignitor having a high firing capacity with reduced frictional flow losses.
A further object of the present invention is to provide a gas pipe ignitor which produces a pilot flame well mixed with air in a controlled zone in which combustion is initiated and sustained. Yet another object of the present invention is to provide a gas pipe ignitor which offers an improvement in the quantity of combustion air available at the same pressure loss as compared with prior art ignitors.
According to one aspect of the present invention, the gas pipe ignitor has a single fuel pipe running through the air supply pipe with the single fuel pipe providing fuel for the pilot flame and for the primary ignitor combustion fuel. A truncated spherical bluff body located in the air supply pipe reduces the flow entrance losses and maintains the necessary downstream turbulence and recirculation zone. The bluff body has a central opening for the fuel pipe and is orificed to provide the desired ratio of pilot gas to primary gas. Integral pilot fuel ports are circumferentially located around the orifice to provide the pilot gas to the truncated face of the bluff body where the pilot gas is evenly distributed by a diffuser ring.